Page 210 - Sedimentology and Stratigraphy

Recognition of Deltaic Deposits 197

only rarely associated with deltas, occurring as accu-
mulations on delta fronts where the supply of clastic
detritus is low: examples in modern settings and from
the stratigraphic record indicate that carbonates form
on deltas in arid and semi-arid environments, where
the supply of clastic sediment to the delta is highly
ephemeral (Bosence 2005).
Palaeontological evidence from fauna and flora
can be important in the recognition of the marine
and continental subenvironments of a delta. A dis-
tinct fauna tolerant of brackish water may be found
near the mouths of channels and in the interdistri-
butary bays where fresh and marine water mix. The
mixture of shallow-marine, brackish and freshwater
fauna plus coastal vegetation is also characteristic of
deltaic environments. The contrast between fresh
and saline water is not present in deltas formed at
the margins of freshwater lakes and in these settings
the recognition of the delta must be based on the
facies patterns.
A final point to emphasise is that the various
models of different types of delta presented in this
chapter are just a few examples of the possible com-
binations of the controls that determine the form and
facies of a delta. Any modern or ancient delta should
be considered in terms of the evidence for the effects
of different factors – sediment grain size, basin water
depth, the relative importance of river, tide and wave
influences – and should not be expected to exactly
match any of the models presented here or any other
text books.
Characteristics of deltaic deposits
. lithologies – conglomerate, sandstone and mudstone
. mineralogy – variable, delta-front facies may be
compositionally mature
. texture – moderately mature in delta-top sands and
gravels, mature in wave-reworked delta-front deposits
. bed geometry – lens-shaped delta channels, mouth-
bar lenses variably elongate, prodelta deposits thin
bedded
. sedimentary structures – cross-bedding and lamina-
tion in delta-top and mouth-bar facies
. palaeocurrents – topset facies indicate direction of
progradation, wave and tidal reworking variable on
delta front
. fossils – association of terrestrial plants and animals
of the delta top with marine fauna of the delta front
Fig. 12.22 A schematic graphic sedimentary log of tide- . colour – not diagnostic, delta-top deposits may be
dominated delta deposits. oxidised